C source.

Cgo first invokes gcc -E -dM on the preamble, in order to find out
about simple #defines for constants and the like. These are recorded
for later use.

Next, cgo needs to identify the kinds for each identifier. For the
identifiers C.foo, cgo generates this C program:

	<preamble>
	#line 1 "not-declared"
	void __cgo_f_1_1(void) { __typeof__(foo) *__cgo_undefined__1; }
	#line 1 "not-type"